Alumina trihydrate, the precursor of many alumina-based products, including calcined alumina used for making metallic aluminum by reduction, is obtained from alumina-containing ores, such as bauxite. Recovery of the alumina content of bauxite is accomplished by the Bayer process. The Bayer process involves the digestion of bauxite ore with a caustic, such as sodium hydroxide, at elevated temperatures and pressures. Digestion of the bauxite results in a saturated sodium aluminate liquor, referred to as "pregnant liquor". Alumina trihydrate is recovered by precipitation, through the addition of seed alumina.
Bauxite is found in many parts of the world and the composition of the ores may vary from place to place. Bauxite ore contains organic impurities. These organic impurities will be co-extracted with the alumina trihydrate during digestion and will contaminate the pregnant liquor.
Most of the organic impurities found in the ores consist of high molecular weight compounds, such as humic acids. These organic impurities decompose to lower molecular weight compounds during the caustic digestion process and produce a whole spectrum of organic salts dissolved in the pregnant liquor. The organic impurities dissolved in the liquor include color-causing compounds, such as humates, and, consequently, the pregnant liquor will usually possess a dark red color. Since the Bayer process involves extensive recycling of the used caustic liquor, the organic impurity content of the liquor will continuously increase. The accumulation of organic impurities can reach high levels and seriously interfere with the production of alumina trihydrate.
Since the control of organic impurity levels in Bayer process liquors is an important facet in the production of alumina trihydrate, several methods have already been developed for such organic impurity level control. It has been suggested in U.S. Pat. No. 4,046,855 (Schepers et al) that organic impurities can be removed from Bayer process liquors by contacting the liquor with a magnesium compound which will form a precipitated mixture of magnesium and aluminum hydroxides.
In U.S. Pat. No. 4,101,629 (Mercier et al), a barium-containing compound is added to Bayer process liquors. The barium compound precipitates as barium aluminate and the precipitated material may also include barium salts of organic impurities present in the liquor.
In U.S. Pat. No. 4,215,094 (Inao et al), a copper-catalyzed wet oxidation process is recommended for the oxidation or organic impurities, followed by addition of a sulfur-containing compound to remove the copper catalyst as a precipitate. In U.S. Pat. No. 4,275,042 (Lever), sodium oxalate, one of the organic impurities in Bayer liquor, is removed from spent Bayer liquor. In the '042 patent, dissolved sodium oxalate is removed from spent Bayer liquor by addition of a cationic sequestrant to the spent liquor. The cationic sequestrant, preferably a simple quaternary nitrogen compound possessing medium and longchain alkyl groups and a single cationic charge, produces an insoluble product with humic compounds present in the liquor, thus destabilizing the spent liquor with respect to sodium oxalate. Destablizing allows precipitation of a portion of the oxalate impurity content. Although the method shown in the Lever patent allows removal of a portion of the organic impurity content of the liquor, the insoluble sequestrant-humic acid product will form an oily scum on the surface of the spent liquor and cannot be readily removed from the spent liquor. Elimination of the oily layer form the surface of the liquor cannot be done by conventional filtration. It has to be accomplished either by equipment adapted for this particular purpose or by using a filtration method capable of dealing with semicolloid surface layers. Consequently, the difficulties associated with the process render it impractical.
In U.S. Pat. No. 4,275,043 (Gnyra), a purification method is described which allows reduction of the oxalate impurity level of spent Bayer process liquor. Removal of oxalate and a limited quantity of humic matter is accomplished by treating the impure spent liquor with an adsorbent, such as activated carbon, activated alumina, or clay.
In U.S. Pat. No. 4,335,082 (Matyasi et al) suggest the removal of organic impurities from impure Bayer liquor by caustifying the liquor with lime, followed by evaporation of the cauticized liquor. A similar purification process is disclosed in U.S. Pat. No. 4,280,987 (Yamada et al). In this process, Bayer liquor is first evaporated, then calcined at high temperature after its alumina and caustic content is adjusted to a predetermined level. This process, known in the Bayer industry as "liquor burning", is an effective means of organic impurity removal.
U.S. Pat. No. 4,578,255 (Roe et al) discloses a process for removing humate-type organic impurities from Bayer process liquors using diallyl dimethyl ammonium chloride polymers having an Intrinsic Viscosity of less than about 1.0 and copolymers having Intrinsic Viscosity of less then about 1.5. Although these low Intrinsic Viscosity polymers are somewhat effective at removing humate-type impurities, the present invention, as detailed below, provides polymers which are much more effective in reducing humate-type impurities. The polymers of the present invention can be dosed at a lower concentration than the polymers of the Roe patent while providing a greater benefit to the Bayer process operator. Accordingly the Bayer process operator obtains a better product at a lower cost than was possible using the methods of the prior art.